Formation of residual ferrite in 9Cr-ODS ferritic steels

It is recognized that the high-temperature strength of 9Cr-ODS ferritic steels is maintained by the presence of a ferrite phase. In order to clarify the formation process of the ferrite phase, 9Cr-ODS ferritic steels containing various contents of Y2O3, i.e., 0 mass%, 0.1 mass%, 0.35 mass%, and 0.7 mass%, are prepared by means of mechanical alloying and hot-pressing. The ferrite phase is formed by the addition of 0.35 mass% and 0.7 mass% Y2O3; however, it is not formed for steels without and with 0.1 mass% Y2O3 normalized for 1 h at 1050 °C. It is considered from the thermodynamic analyses that the pinning of the α–γ interface motion by the dispersed Y–Ti complex oxide particles can be attributed to the retention of the ferrite phase normalized at 1050 °C, and this ferrite phase cannot be δ-ferrite that is in equilibrium with the γ-phase.